By Joe Ikley and Bill Johnson, Purdue University Extension Specialists

One of our biggest weed problems across Indiana every year is control of marestail in soybeans, and 2017 has been no different. Many growers had a lot of trouble controlling marestail in their spring burndown programs in April and May.

There were two distinct times this spring when our phone lines and emails lit up with questions about poor marestail control.

First, we had two heavy frost events on April 8th and 9th, and a lot of fields that got sprayed those days and the following one to two days experienced poor control of marestail. Plants that receive frost damage will slow or shut down their growth for a few days until warmer temperatures and better growing conditions return.

We will get our best weed control with systemic herbicides such as glyphosate, 2,4-D, or dicamba when these weeds are actively growing. Many of the fields that were sprayed saw poor results due to using these systemic herbicides on marestail that was not actively growing while plants were trying to recover from the frost events.

In many cases, waiting an extra day or two until daytime temperatures were in the 50’s would have helped increase marestail control.

The second wave of questions rolled in after the heavy rain and cold temperatures in early May gave way to sunny and warm days that finally allowed equipment traffic in many fields.

By this point marestail was often well over a foot tall, a height that limits the effectiveness of any burndown program. In our minds, the issues this spring highlighted the importance of using a fall applied herbicide program to control marestail and other winter annual weeds.

Marestail

Now is the time to start thinking about fall applied herbicides to control marestail and other winter annual weeds in corn and soybean production. When harvest and post-harvest conditions allow, fall is the best time to control many of these weeds. This is because the weeds are a lot smaller in the fall, and our fall weather tends to be consistently warmer and drier than our variable cool and wet springs.

Marestail is controlled effectively by many fall applied programs. Managing this weed with fall applied programs should be strongly considered in areas with known infestations of glyphosate-resistant populations. All of the programs listed below will provide control or suppression of glyphosate-resistant marestail except glyphosate alone.

The addition of 2.4-D to these products will greatly increase control of glyphosate-resistant populations.

Dandelion

There are pockets across the state that also deal with heavy infestation of dandelions every year. Dandelions are controlled much more effectively with fall applied programs than with spring applied herbicides. All programs listed will provide control or suppression of dandelion.

The key in most of the programs is the use of 2,4-D or glyphosate. Control of dandelion with 2,4-D is rate responsive. If dandelion infestations warrant fall treatment, use 2,4-D alone or with the other products at 1 qt/A unless you are using Canopy XL. If using Canopy XL, then the 2,4-D rate can be reduced to 1 pt/A.

Glyphosate is much more effective on dandelion in the fall than in the spring. Use 22 oz/A of Roundup Powermax, or 32 oz/A of the other 3 lb acid equivalent/gallon formulations.

Other Winter Annual Weeds

In addition to marestail and dandelion, the other most common winter annual weeds found in Indiana are henbit, purple deadnettle, chickweed, and cressleaf groundsel. In Table 1, we have listed the commercially available programs that have provided 80% or more control of these weeds in University research trials.

Most of the commercially available programs offer flexibility in growing either corn or soybean in the following cropping season. This is because their rotational intervals on the label are shorter than the time frame between a fall application and traditional planting dates in April and May of the following year.

Specific programs mentioned below that do not offer crop rotation flexibility include Canopy XL (must grow soybean in 2018), and Princep or simazine (must grow only corn in 2018).

Application Timing and Treatments

Fall applied herbicide treatments can be applied from mid October until early December. Products that do not have appreciable residual activity in soil (glyphosate, 2,4-D, Gramoxone alone) should be applied late enough so that weed reemergence after application is minimal.

However, foliar activity with these products is better when applications are made during a period of several days of daytime air temperatures of 50 degrees or greater. The addition of metribuzin to these burndown programs will provide an additional 2 to 4 weeks of residual activity in the fall. This added residual from metribuzin is especially beneficial in fields with high populations of marestail.

For dandelion control, applications should be about the time of the 1st killing frost (typically late October to early November). Products with residual activity in the soil have more flexibility in application timing and can be applied as soon as the crop is removed from the field.

This table gives a general comparative rating of “burndown” herbicides used in no-till corn and/or soybean production. Under unfavorable conditions, some herbicides may not perform as well as indicated below. Under very favorable conditions, control may be better than indicated.

Herbicide rate, weed size and stage of growth, and environmental conditions interact to inﬂuence herbi- cide performance. Weed control rating: 9 = 90% to 100%; 8 = 80% to 90%; 7 = 70% to 80%; 6 = 60% to 70% control; and – = less than 60% control, not recommended. Ratings are for control of existing vegetation only (not residual control). Treatments containing glyphosate should be applied with AMS (and surfactant if required by the glyphosate product label).

Most other treatments should be applied with a COC or MSO (plus UAN if recommended by the label). Ratings for 2,4-D are based on a rate of 0.5 lb ae/A unless otherwise indicated – increasing the 2,4-D rate to 1.0 lb ae/A will improve control of legumes, dandelion, marestail, and some other weeds.